.Net: Allow Kernel to be mutable by AgentChatCompletions

[RogerBarreto]

Motivation and Context

• Fixes .Net: Bug: Kernel.Data no longer retains information across plugins since v1.54 #12534

• This regression seems to be part of .Net: Add AIContextProvider support to Semantic Kernel #11689 where kernel instance was is cloned prior agent iterations, as I didn't captured any failing unit tests for the added AIContext providers considering this as a valid fix.

• Added UT covering the expected Kernel mutability.

[markwallace-microsoft]

@RogerBarreto is this integration test failure related to your changes?

[xUnit.net 00:05:48.23]     SemanticKernel.IntegrationTests.Connectors.AzureOpenAI.AzureOpenAIChatClientRequiredFunctionChoiceBehaviorTests.SpecifiedInPromptInstructsConnectorToInvokeKernelFunctionAutomaticallyAsync [FAIL]
[xUnit.net 00:05:48.23]       Polly.Timeout.TimeoutRejectedException : The operation didn't complete within the allowed timeout of '00:00:30'.
[xUnit.net 00:05:48.23]       ---- System.Threading.Tasks.TaskCanceledException : The operation was canceled.
[xUnit.net 00:05:48.23]       -------- System.IO.IOException : Unable to read data from the transport connection: Operation canceled.
[xUnit.net 00:05:48.23]       ------------ System.Net.Sockets.SocketException : Operation canceled